Method of making cubic zinc sulfide with copper



United States Patent 3,380,841 METHOD OF MAKING CUBIC ZINC SULFIDE WITH COPPER Chester D. Beintema, Vista, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware No Drawing. Filed Feb. 5. 1965, Ser. No. 430,762

3 Claims. (Cl. 117-201) ABSTRACT OF THE DISCLOSURE Method of forming cubic zinc sulfide by mixing zinc sulfide .powder with copper nitrate and then heating the mixture in vacuum to the temperature which zinc sulfide vaporizes so that the zinc sulfide and copper deposit upon the substrate.

This invention relates to the formation of a material exhibiting bombardment induced conductivity for use in direct-viewing storage tubes. More particularly, the in vention relates to a process in which the formation of zinc sulfide in the cubic crystal structure form thereof is enhanced and stabilized while transformation to the hexagonal form is inhibited.

It is known that certain types of cathode ray tubes in which visual displays may be stored and presented for relatively long periods of time utilize a storage target which functions by means of the phenomenon of bombardment induced conductivity. Such a storage display tube is described by N. H. Lehrer in U.S. Patents 3,086,- 139 and 3,089,055. In the latter patent it is noted that of the three types of zinc sulfide (cubic, amorphous, and hexagonal), only cubic zinc sulfide appears to possess the desired properties suitable for use in such display tubes and a rather extensive process is described in the former patent for insuring the formation and stabilization of zinc sulfide in the cubic crystal form thereof, including an aging period lasting for several days.

It is an object of the present invention to provide an improved method for forming cubic zinc sulfide.

Another object of the invention is to provide an improved method for vapor-depositing zinc sulfide in the zinc sulfide while inhibiting the transformation thereof to the hexagonal form.

Another object of the invention is to provide an improved method for vapor--depositing zinc sulfide in the cubic form thereof on a substrate.

In an article entitled The Influence of Copper on Structural Transformations in ZnSzSu, Cl, published in December 1963 in the Journal of the Electrochemical Society, it is reported that the addition of copper to crystalline zinc sulfide enhances the formation of the cubic crystal form thereof and stabilizes this structure. On the other hand, it has been noticed that the presence of chlorine atoms tends to encourage the formation of hexagonal zinc sulfide. Until the present invention, most of the usual methods employed to introduce copper in zinc sulfide have involved the use of copper chloride which introduces the unwanted chlorine atoms. Other attempts to co-evaporate pure copper onto a substrate with zinc sulfide have been beset with difiiculties, chief of which has been the inaccuracy of the copper deposition.

The objects of the present invention are realized, while avoiding the difficulties of other methods of the prior art, by saturating zinc sulfide powder with copper nitrate and then drying the powder. Thereafter, the zinc sulfide is heated and vapor-deposited onto a desired substrate. Apparently the zinc sulfide and the copper nitrate evaporate together with the copper nitrate undergoing a complex reaction including decomposition to yield oxides of nitrogen and oxygen itself, with the copper continuing on with the zinc sulfide to the surface to be coated therewith. In this manner, the zinc sulfide is readily and easily doped with pure copper in known quantity and with repeatable accuracy.

It has been found that the desired amount of copper in the zinc sulfide to best enhance the formation of the cubic structure thereof is between 0.01% to 0.1% by weight of copper to zinc sulfide. This is achieved by first dissolving copper nitrate, Cu(N0 -3H O, in water in a quantity calculated to yield from 0.01% to 0.1% of copper by weight reference to the weight of the zinc sulfide to be doped. Thus, for each gram of zinc sulfide to be doped the actual quantity of copper nitrate is from 0.4 to 4 milligrams.

The zinc sulfide, in finely powdered form, is added to the copper nitrate solution and stirred well. The water is then caused to evaporate as by heating the solution in an oven at a temperature of C., for example. The resulting solid residue of ZnS and Cu(NO -3H O (hereinafter called the charge) is then placed in an evaporation vessel such as a tantalum boat adapted to be heated. The evaporation boat with its charge is then placed in suitable apparatus for causing the zinc sulfide to evaporate and deposit on a suitable substrate. Thus, the charged evaporation boat and a substrate of nickel, for example, may be placed in a bell jar with the substrate being positioned over the evaporation boat. The bell jar may then be evacuated, typically to a pressure of 10- mm. Hg, and the evaporation boat is heated to a temperature of from 1100 to 1200 C., for example. It is believed that the following reactions occur during this step of the process:

with the Water and nitric acid as formed continuously pumped out as vapor.

with the oxygen also being pumped out leaving only the copper remaining to dope the zinc sulfide.

The copper and zinc sulfide co-evaporate at the temperature range stated and continue on to the surface of the substrate to be coated with zinc sulfide. By maintaining the substrate at a lower temperature than the vaporization temperature of zinc sulfide, deposition of the zinc sulfide and copper onto the substrate is achieved. The thus-deposited and copper-doped zinc sulfide is invariably found to be in the desired cubic structure and its stability is much enhanced in comparison with thin films of cubic zinc sulfide formed by other techniques. It will be appreciated that films of any thickness may be formed by the vapor-deposition process described by allowing the process to continue until the desired thickness is attained.

There thus has been described an improved process for forming films of cubic zinc sulfide by incorporating predetermined amounts of copper therein which enhances the formation and stability of zinc sulfide in the cubic structure.

What is claimed is:

1. The method of preparing cubic zinc sulfide comprising: heating a mixture of zinc sulfide and copper nitrate in vacuum to the temperature at which said zinc sulfide va orizes, vaporizing said zinc sulfide, and depositing said zinc sulfide and copper upon a substrate.

2. The method of preparing cubic sulfide comprising: mixing zinc sulfide with a solution of copper nitrate, drying said zinc sulfide and copper nitrate to yield a mixture thereof, heating said mixture in vacuum to the temperature at which said zinc sulfide vaporizes, vapor- 3 4 izing said zinc sulfide, and depositing said zinc sulfide References Cited and copper upon asubstrate. UNITED STATES PATENTS 3. The method of preparing cubic zinc sulfide comprising: mixing zinc sulfide with a solution of copper gusano et nitrate containing from 0.01% to 0.1% by weight of 5 29O8594 10/1959 r et H 201 copper with respect to the amount in weight of zinc 3O86139 4/1963 f sulfide, heating said zinc sulfide and copper nitrate to 3089O55 5/1963 Lehrer yield a dry mixture thereof, heating said dry mixture to 3:22:21! 12/1965 Lagos at least 1100 C. in vacuum to thereby vaporize said zinc sulfide, and depositing said zinc sulfide and copper l0 ALFRED LEAVITT, Primary Examinerupon a substrate. A. GOLIAN, Assistant Examiner. 

